WinCOFF: Add support for -ffunction-sections

This is a pretty straight forward translation for COFF, we just need to
stick the function in a COMDAT section marked as
IMAGE_COMDAT_SELECT_NODUPLICATES.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@204565 91177308-0d34-0410-b5e6-96231b3b80d8